In the integrated circuit (IC) industry, input buffer circuitry is fabricated on a periphery of an integrated circuit die and electrically connected between an external pin of the integrated circuit and internal circuitry within the IC. In essence, the input buffer circuitry is an interface between the internal IC circuitry and an external environment outside of the IC so that data can be communicated from the external environment in to and out from the integrated circuit. Integrated circuits (ICs) are routinely designed such that one integrated circuit in an electrical system operates at a first power supply voltage and a second integrated circuit operates using a different power supply voltage level. For example, a first common voltage supply in the industry is roughly a 5 volt voltage supply, a second voltage supply used in the industry is roughly 3.3 volts, a third voltage commonly used in the industry is roughly 2.5 volts, and a fourth commonly used voltage supply level is roughly 1.8 volts where any electrical system may contain one or more devices operating at these voltage levels. As an example, for example, a 5 volt part will need to interface to a 1.8 volt part wherein the input buffer which is used to communicate between these two parts must be able to handle the discrepancy in voltage while still rendering acceptable performance. Due to the fact that there are at least four different common power supply voltage levels which are readily available in the industry, communication between these different devices has become more complex. Input and/or output buffer must now ensure interoperability of these different devices while maintaining optimal performance, if possible. Therefore, the design of such buffers has become increasingly more difficult and increasingly more important in the IC industry.
FIG. 1 illustrates a prior art input buffer 500 that is commonly used in the integrated circuit industry. The buffer of FIG. 1 is fabricated on an IC die and allows two integrated circuits with different power supply voltages to interface to one another in a fairly efficient manner. The integrated circuit incorporating the circuit 500 contains a chip pad 512 in FIG. 1 which is used to receive input data from external to the integrated circuit. An input signal provided to the chip pad/terminal 512 will pass through a resistive element 510 and be communicated through an input transistor 504 illustrated in FIG. 1. The transistor 504 of FIG. 1 has a gate/control electrode that is coupled to the outside VDD level (OVDD) 514. The OVDD signal 514 is the power supply voltage level used by the external peripheral that is coupled to the pad 512 and providing the data into the circuit 500. The OVDD signal 514 is provided into the integrated circuit via another pin coupled to the integrated circuit where the OVDD is not specifically illustrated in FIG. 1. As a typical example, OVDD may be one of 5 volts, 3.3 volts, 2.5 volts, and 1.8 volts in most devices that use complementary metal oxide semiconductor (CMOS) silicon logic devices.
The transistor 504 ensures that the node 505 does not rise in voltage to a damaging voltage level that can harm the transistors 508 and 506. Specifically, any voltage provided on the chip pad 512 through the resister 510 will drop at least a threshold voltage (Vt) in magnitude when communicated through the transistor 504 whereby the voltage on node 505 should be less than OVDD when OVDD 514 in FIG. 1 is greater than VDD 516. In short, transistor 504 will protect the transistors 508 and 506 from a damaging overvoltage occurrence that may occur when an integrated circuit operating at a first power voltage is coupled to another integrated circuit operating at a different second power supply voltage.
The input signal initially provided through the chip pad 512 is then provided via the node 505 to the inverter comprising transistors 506 and 508. The inverter, comprising transistors 506 and 508, is connected to a ground potential 520 and an internal VDD voltage 516. The VDD voltage 516 is a voltage that is supplied to operate all the circuitry on the integrated circuit including the input buffer 500. Typically, VDD 516 can be any voltage but is usually 2.5 volts or 1.8 volts in modern high performance low power microprocessors and memory. The inverter, comprising the transistors 506 and 508, will buffer the input signal to an internal node 518 with a logical inversion. This inverted signal provided by transistors 506 and 508 is routed to functional located within the integrated circuit containing the circuit 500 so that incoming information may be processed by the system.
In case where OVDD 514 minus a threshold voltage (OVDD-Vt) is substantially less than VDD 516 in voltage, the transistor 502 is provided. Transistor 502 will pull the node 505 up to an acceptable voltage during certain conditions to shut off the inverter comprising transistors 506 and 508 when (OVDD-Vt) is substantially less than VDD. Therefore, transistor 502 ensures that, when a substantial mismatch of voltage occurs between OVDD and VDD, the inverter containing transistors 506 and 508 can be completely turned off regardless of this extreme difference in voltage.
While the circuit of FIG. 1 is commonly used and is an adequate output buffer in most circumstances, the circuitry of FIG. 1 has many disadvantages. First, due to the difference voltages OVDD 514 and VDD 516, the inverter comprising transistors 506 and 508 is typically fixed to a trigger point that is directly a function of specific OVDD and VDD voltage values. For example, if OVDD is 5 volts and VDD is 3.3 volts, the trigger voltage of the inverter comprising gates 506 and 508 is statically fixed to roughly 2.5 volts, which is not half way between VDD and ground, when the devices 508 and 506 are matched. This is not advantageous since the inverter now has a trigger point that is not roughly half way between VDD 516 (3.3 volts) and ground 520. To compensate for this noise margin problem, the transistors 508 and 506 are fabricated with significantly different in aspect ratios to statically fix the trigger point at yet another voltage value (e.g., 1.6 volts). This mismatching of the transistors 506 and 508 will result in an imbalanced and non-symmetrical inverter that will have different operating characteristics when the inverter is transitioning from a high voltage to a low voltage and vice versa. Since timing constraints of external buses and the like are typically designed to the worse case transition, the mismatch in the transistors 506 and 508 to correct noise margins may impact the maximal speed at which the device can be operated.
In addition, since the trigger point of FIG. 1 is statically fixed, the circuitry of FIG. 1 can only function optimally when OVDD 514 and VDD 516 are known quantities that will not change for different applications. If a designer wants to have flexibility to change VDD 516 to another voltage and/or to change OVDD 514 to another voltage, use the IC in a different application, or add different peripherals to the system having different OVDDs, then the statically designed circuit 500 of FIG. 1 will not compensate for these changes in OVDD whereby the trigger point will not be properly set. Improperly set trigger points will greatly reduce the speed of operation of the circuit and may, in some circumstances, render the circuit of FIG. 1 completely inoperable. For example, in the circuitry of FIG. 1, if (OVDD-Vt) were to be substantially less than VDD, the circuitry of FIG. 1 may not even be able to pass a logic 1 from the chip pad 512 to the internal gates 518. In this case, the circuitry of FIG. 1 is completely nonfunctional. In essence, since the trigger point of FIG. 1 is only a function of VDD and not OVDD and that trigger point can be greater than (OVDD-Vt), flexibility in system level design by changing VDD and/or OVDD is greatly reduced and multiple ICs must be custom designed for differing VDD and OVDD combinations.
In addition, in some OVDD and VDD cases, the circuitry of FIG. 1 will result in a trigger point that is located where the noise margins that result are not adequate. In these cases, erroneous signals or great degradation in performance may occur whereby the circuitry of FIG. 1 is not desirable.
Therefore, a need exists in the industry for an input buffer that has one or more of: (1) a balanced output inverter structure; (2) a trigger point that can varies only as a function of OVDD 514); (3) a trigger point designed to have optimal noise margins; and/or (4) the ability to always pass functional high and low logic values regardless of the relationship between OVDD and VDD.